The present invention relates to a method for controlling a stepping motor, and more particularly, to a method for controlling a tracking servo of a stepping motor used in a disc reproduction apparatus.
A digital audio compact disc (CD) may be used as a read-only memory (CD-ROM) for various types of computer-readable digital data. A disc reproduction apparatus that reproduces such a CD has a servo mechanism to control the relative position between the disc and a pickup so that the pickup can correctly trace a spiral recording track on the disc.
FIG. 1 is a schematic block diagram of a disc reproduction apparatus 100. A spiral recording track is formed on at least one side of the disc 1. Digital data, which complies with a predetermined format, is recorded along the recording track. For example, in a CD, pits having a predetermined length are formed on the recording track in accordance with an EFM signal, which is obtained by performing eight to fourteen modulation (EFM) on digital data. The spindle motor 2 rotates the disc 1 at a predetermined speed in accordance with a drive signal SD, which is provided from a servo control circuit 7.
A pickup 3, which is mounted on a sled 4, includes a laser beam source and a sensor. In a state in which the sled 4 is attached to a drive device (not shown) and the pickup 3 is opposed to the recording track surface of the disc 1, the pickup 3 is moved in the radial direction of the disc 1. An actuator 5 moves the sled 4 in the radial direction of the disc 1 in accordance with a drive signal TD provided from a servo control circuit 7.
A signal processing circuit 6 receives an output signal read from the disc 1 by the pickup 3 and performs operations such as waveform shaping and binary coding to generate the EFM signal. The EFM signal is shifted between two levels in accordance with the pits and lands formed on the recording track of the disc 1. Based on the output signal of the pickup 3, the signal processing circuit 6 generates a tracking error signal TE and an off track signal OT. More specifically, the pickup 3 includes a main beam source, which reads the data recorded on the disc 1, and an auxiliary beam source, which reads the position of the main beam source relative to the recording track of the disc 1. The signal processing circuit 6 generates the EFM signal from the read output signal of the main beam source and generates the tracking error signal TE from the read output signal of the auxiliary beam source. The tracking error signal TE is normally maintained at xe2x80x9c0xe2x80x9d level when the reading position of the pickup 3 relative to the disc 1 is correct. When the pickup 3 moves toward the inner side or the outer side of the disc and away from the proper position, the error signal TE shifts to a negative or positive polarity level. The signal processing circuit 6 generates the off track signal OT from a low frequency component of the EFM signal. The off track signal OT is maintained at a low level when the EFM signal is output properly (i.e., when the pickup 3 correctly reads data from the recording track of the disc 1). When the pickup 3 moves away from the proper position and the EFM signal is thus not properly output, the EFM signal goes high.
The servo control circuit 7 receives the EFM signal, the tracking error signal TE, and the off track signal OT from the signal processing circuit 6 and generates a spindle motor drive signal SD and an actuator drive signal TD. The spindle motor drive signal SD is generated to keep the cycle of the EFM signal constant when, for example, performing constant linear velocity (CLV) control to drive the disc 1 at a constant linear velocity. Further, the spindle motor drive signal SD is generated in accordance with a reference clock signal having a constant frequency when performing constant angular velocity (CAV) control to drive the disc 1 at a constant angular velocity. The servo control circuit 7 generates the actuator drive signal TD based on the tracking error signal TE and the off track signal OT. For example, to read data from the disc 1, the actuator drive signal TD is generated so that the tracking error signal TE becomes close to the xe2x80x9c0xe2x80x9d level. Such servo control rotates the disc 1 at a predetermined velocity (constant linear velocity or constant angular velocity) while the pickup 3 traces the recording track on the disc 1 to appropriately read the data recorded on the disc 1.
The actuator 5 causes the pickup 3 and the sled 4 to take a short jump or a long jump. A short jump moves the pickup 3 within a movable range on the sled 4. A long jump moves the sled 4 with the position of the pickup 3 in a fixed state.
In a short jump, only the pickup 3 is moved with the sled 4 in a fixed state. When the pickup 3 approaches the limit of the movable range on the sled 4, the sled 4 is moved slightly so that the pickup 3 may further be moved on the sled 4. The short jump is taken when reading data as the pickup 3 follows the recording track of the disc 1.
In a long jump, the moving distance of the pickup 3 relative to the disc 1 (normally, longer than the movable range of the pickup 3) is set, and the sled 4 moves in accordance with the set moving distance. In this state, the pickup 3, which is substantially fixed on the sled 4, reads signals from the disc 1 as the pickup 3 traverses the recording track on the disc 1. The reading operation of the pickup 3 is performed to count the number of recording tracks the pickup 3 skips and not to read the data recorded on the disc 1.
The polarity of the tracking error signal TE is inverted whenever the pickup 3 traverses the single recording track. Thus, the generated tracking error signal TE has a sine waveform when the pickup 3 consecutively traverses the recording track a number of times. Further, the generated EFM signal has a predetermined amplitude when the pickup 3 is above the recording track, and the generated EFM signal has a fixed value when the pickup 3 moves away from the recording track. Thus, the off track signal OT, which is generated from the low frequency component of the EFM signal alternately goes high and low as the pickup 3 traverses the recording track. The off track signal OT has the same cycle as the tracking error signal TE and has a phase that is either advanced or delayed by n/2 from the tracking error signal TE with respect to the moving direction of the pickup 3 (sled 4). The number of recording tracks the pickup 3 skips is detected by counting the tracking error signal TE or the off track signal OT. The movement of the sled 4 is stopped when the pickup skips the number of recording tracks that correspond to a target distance.
When moving the sled 4 by a target distance in the radial direction of the disc 1, the drive signal TD is generated so that the sled 4 is accelerated when the movement starts and the sled 4 is decelerated just before the position where the sled 4 is to stop. For example, referring to FIG. 2, from time t0, when the movement of the sled 4 starts, to time t1, when the sled 4 reaches a predetermined speed, the sled 4 is moved at a constant acceleration rate. Then, from time t1 to time t2, the sled 4 is moved at a constant velocity. After time t2, the sled 4 is moved at a negative acceleration rate so that the sled 4 stops at time t3. Time t2 is determined from the moving distance of the sled 4 and the acceleration rate during deceleration. When the moving distance of the sled 4 is relatively short or when the absolute value of the acceleration rate during deceleration is relatively small, time t2 is set so that it is earlier than time t1. In this case, constant velocity movement is not performed, and acceleration is switched to deceleration at time t2.
When moving the sled 4 with the drive signal TD, servo control is performed while confirming the moved distance of the sled 4 on the bases of the count value of the number of recording tracks the pickup 3 (sled 4) skips. Therefore, the sled 4 may stop after exceeding the target moving distance due to factors such as circuit delays or mechanical delays of a moving mechanism. Further, servo control, which is performed by confirming the actual moved distance of the sled 4, hinders high-speed movement of the sled 4.
To solve the above problem, a stepping motor enabling accurate rotation control may be employed in lieu of the spindle motor 2 to perform servo control. When using such a stepping motor, the moved distance of the sled 4 may easily be confirmed, and the sled 4 may be stopped accurately. However, to drive the stepping motor, a complicated drive pulse signal must be generated. Further, when repeating smooth acceleration and deceleration, the load applied to a drive circuit increases.
A perspective of the present invention is a method for controlling a stepping motor that moves an object from an initial position by a target distance. The method includes the steps of calculating a moved distance of the object from the difference between an initial position and present position of the object, calculating a remaining distance from a difference between the target distance and the moved distance, updating a first velocity value by adding a first acceleration value, which increases in a stepped manner in accordance with the moved distance, to the first velocity value until the moved distance exceeds a first reference value, updating a first position value, which indicates the present position of the object that is accelerated, by adding the updated first velocity value to the first position value, updating a second velocity value when the remaining distance is less than a second reference value by subtracting a second acceleration value, which decreases in a stepped manner in accordance with the remaining distance, from the secured velocity value, and updating a second position value, which indicates the present position of the object that is decelerated, by adding the updated second velocity value to the second position value.
A further perspective of the present invention is a method for controlling a stepping motor that moves an object from an initial position to a target position. The method the steps of includes calculating a moved distance of the object from the difference between an initial position and present position of the object, calculating a remaining distance from a difference between a target distance and the moved distance, comparing the remaining distance with a reference value, updating a first velocity value by adding a first acceleration value to the first velocity value when the remaining distance is less than the reference value, updating a first position value, which indicates the present position of the object that is accelerated, by adding the updated first velocity value to the first position value, repeating the step for calculating the moved distance, the step for calculating the remaining distance, the step for comparing the remaining distance with the reference value, the step for updating the first velocity value, and the step for updating the first position value until the remaining distance exceeds the reference value, updating a second velocity value when the remaining distance exceeds the reference value by subtracting a second acceleration value from the second velocity value, updating a second position value, which indicates the present position of the object that is decelerated, by adding the updated second velocity value to the second position value, and repeating the step for calculating the moved distance, the step for calculating the remaining distance, the step for comparing the remaining distance with the reference value, the step for updating the second velocity value, and the step for updating the second position value until the remaining distance becomes zero.
Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.